1. Field of the Invention
The present invention relates to a cellular communication system and, in particular, to a physical channel communication method and apparatus for random access in a cellular communication system including a plurality of relay nodes and user equipments using the same system information.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted into many communication and broadcast systems. OFDM is robust against multipath fading and intersymbol interference, guarantees orthogonality among intra-cell users, and allows for an efficient use of the available radio frequency spectrum. Due to these advantageous factors, OFDM is a strong candidate for high speed broadband wireless communications as compared to Direct Sequence Code Division Multiple Access (DS-CDMA).
FIG. 1 is a diagram illustrating an OFDM-based downlink frame structure in Evolved Universal Terrestrial Radio Access (EUTRA) specified in the 3rd Generation Partnership Project (3GPP) standards. Referring to FIG. 1, the 20 MHz system bandwidth 101 is divided into 100 Resource Blocks (RBs) 105. An RB consists of 12 consecutive subcarriers 103 by 14 OFDM symbol periods. Each subcarrier 103 for one OFDM symbol duration carries a modulation symbol of downlink channel. Each box within the resource grid representing a single carrier for one symbol period is referred to as a Resource Element (RE) 106. In FIG. 1, the RB is composed of total 168 REs (14 OFDM symbols×12 subcarriers). A single downlink data channel can be assigned one or more RBs for one OFDM symbol duration 104 according to the data rate.
In the cellular communication system, bandwidth scalability is one of the key performance attributes for providing high speed wireless data service. For instance, the Long Term Evolution (LTE) system supports various bandwidths of 20 MHz, 15 MHz, 10 MHz, 5 MHz, 3 MHz, and 1.4 MHz as shown in FIG. 2. Accordingly, the LTE service provider can select one of the available bandwidths, and a mobile terminal also can be configured to support various capacities of 1.4 MHz to 20 MHz bandwidth. In order to fulfill International Mobile Telecommunication (IMT)-Advanced requirements, LTE-Advanced (LTE-A) supports carrier aggregation to allocate up to 10 MHz.
In the system supporting the bandwidth scalability, the mobile terminal is required to be able to carry out the initial cell search without information on the system bandwidth. The mobile terminal can acquire synchronization with the base station and cell ID for demodulation of data and control information through a cell search procedure. The system bandwidth information can be acquired from the Synchronization CHannel (SCH) in the cell search procedure or by demodulating a Broadcast CHannel (BCH) after the cell search procedure. The BCH is a channel used for transmitting the system information of the cell, which the mobile terminal accesses, and is demodulated first right after the cell search procedure. The mobile terminal can acquire the system information such as the system bandwidths, System Frame Number (SFN), and physical channel configuration of the cell by receiving a shared control channel.
FIG. 2 is a diagram illustrating an exemplary frequency resource mapping of SCH and BCH according to a system bandwidth in a conventional system supporting bandwidth scalability. The mobile terminal performs cell search on the SCH and, once the cell search has completed successfully, acquires the system information on the cell through the BCH. In FIG. 2, the horizontal axis 200 denotes frequency in MHz, and the SCH 204 and BCH 206 having 1.08 MHz bandwidth are transmitted in the middle of the system bandwidth regardless of the bandwidth scale. Accordingly, the mobile terminal can find the Radio Frequency (RF) carrier 202 regardless of the bandwidth scale of the system and acquire an initial synchronization to the system by performing the cell search on the SCH 204 defined by 1.08 MHz bandwidth centering on the RF carrier 202. After finding the cell, the mobile terminal demodulates the BCH 206 transmitted within the same 1.08 MHz bandwidth centering on the RF carrier 202 to acquire the system information.
FIG. 3 is a diagram illustrating a frame format of a 10 ms radio frame of an LTE system in which the SCH and BCH are transmitted. The SCH is transmitted in the forms of Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) on every 0th subframe (subframe #0) and every 5th subframe (subframe #5). Each of the PSS and SSS has a length equal to an OFDM symbol duration and is transmitted through 1.08 MHz bandwidth in the middle of the system bandwidth 303 as shown in FIG. 2. The BCH 302 is transmitted for four OFDM symbol durations within the subframe #0.
In the LTE system, the base station (an evolved Node B or eNB) transmits Physical Downlink Control CHannel (PDCCH) information to the mobile station (a User Equipment or UE) about the resource allocation, i.e. Physical Downlink Shared CHannel (PDSCH). In the LTE system, 1, 2, or 3 OFDM symbols at the beginning of a subframe are used for the PDCCH and the remaining OFDM symbols are used for the PDSCH. The reason that the PDCCH is located in the first OFDM symbols of the subframe is that the UE can reduce the power consumption by employing a micro sleep mode for the PDSCH region when no PDSCH is allocated. The UE needs to perform a number of PDCCH blind decodings to a group of PDCCH candidates for control information. In the blind decoding scheme, the eNB transmits the PDCCH using one of multiple control channel candidates, and the UE performs decoding without information on which a control channel candidate is used for the transmission of the control channel. The success or failure of the blind decoding depends on the Cyclic Redundancy Check (CRC) result.
FIG. 4 is a diagram illustrating a principle of uplink resource allocation resource allocation for transmitting a random access preamble in an LTE system. Referring to FIG. 4, the Physical Uplink Control CHannel (PUCCH) is transmitted on a reserved frequency region 402 at the edges of the total available bandwidth in the uplink, and 6 consecutive Resource Blocks (RBs) 403 of the rest part of specific subframes 401 are allocated for the transmission of random access preamble without overlapping with the PUCCH region. This is called Physical Random Access CHannel (PRACH). The PRACH region allocation information for the random access preamble transmission is provided by means of the system information transmitted by the eNB.
In an LTE system, the UE receives the system information (including a set of preamble sequences, resource allocation information for the preamble transmission, Random Access Radio Network Temporary Identifier (RA-RNTI), and preamble transmission power) transmitted by the eNB, and performs random access based on the system information.
In a first step: The UE selects a preamble sequence (hereinafter referred to as message 1) from the preamble sequence set and transmits the selected preamble sequence to the eNB on a random PRACH in the frame.
In a second step: The eNB transmits a message (hereinafter referred to as message 2) containing the corresponding preamble sequence index, Temporary Radio Network Temporary Identifier (Temp-RNTI), and uplink scheduling information on the PDSCH. The PDSCH is indicated using the PDCCH having a CRC scrambled with the RA-RNTI.
In a third step: The UE acquires the PDCCH using the RA-RNTI and receives message 2 from the PDSCH based on the PDCCH. If the sequence index of message 2 and the sequence index of message 1 are identical to each other, the UE transmits a message (hereinafter referred to as message 3) containing the UE ID, contention resolution information, and scheduling request information to the eNB based on the uplink scheduling information acquired from message 2. If the sequence indices are not identical to each other, the UE performs the first step again.
In a fourth step: The eNB transmits a contention resolution message (hereinafter referred to as message 4) to the UE on the PDSCH by using the UE ID acquired from message 3. The PDSCH allocation information is indicated by the PDCCH having a CRC scrambled with the Temp-RNTI transmitted in message 2. The UE checks the PDCCH by using the Temp-RNTI and receives message 4 by using the PDCCH. The UE compares the IDs of message 3 and message 4 and, if the IDs are identical with each other, transmits an ACKnowledgement (ACK) message to the eNB and configures the Temp-RNTI as Cell Radio Network Temporary Identifier (C-RNTI). If the IDs are not identical to each other, the UE performs the first step again.
LTE-Advanced (LTE-A) requires new technologies to support the bandwidth wider than that of the LTE system. The basic obstacle to the high speed data transmission using limited resources is the signal distortion caused by path loss. In order to overcome the path loss problem, a wireless relay technology has been introduced. The wireless relay technology compensates for the path loss of the transmission signal by placing a wireless relay node between the transmitter and the receiver so as to improve the reception performance of the UE, especially at the cell boundary, and increase the system coverage area.
In the case where the wireless relay node receives and transmits signals simultaneously, the transmission signal is likely to interfere the reception signal. Accordingly, the relay node is required to separate the transmission and reception links. There are two techniques for separating the transmission and reception links.
The first technique is to utilize Time Division Duplex (TDD) which separates the transmission and reception links in time over the same frequency band. The second technique is to utilize Frequency Division Duplex (FDD) which separates the transmission and reception links by using different frequency resources at the same time. In FDD, there is a relatively wide frequency offset between the transmission and reception bandwidths to avoid the interference between them, which translates into bandwidth waste. Accordingly, the TDD is preferable for the asymmetry transmission.
In the meantime, the wireless relay system classifies the relay nodes into four types:
Layer 0 Relay (L0 relay): amplifies and then forwards all of the received signal;
Layer 1 Relay (L1 relay): amplifies and then forwards the received signal has other function in addition to that of the L0 relay;
Layer 2 Relay (L2 relay): modulates, decodes, encodes, and then forwards received signal; and
Layer 3 Relay (L3 relay): has base station functions in addition to the relay function.
In the case where the system using the relay nodes other than the L3 relay node, all the UEs can perform random access using the same system information. At this time, the eNB cannot identify which relay node is used for the random access attempt of each UE. Accordingly, the eNB cannot determine the reception timing of the physical channel related to the random access in consideration of the propagation latency of the UE attempting the random access via a relay node. There is, therefore, a need to develop a method for identifying the relay node which the UE uses for random access.